In any situation where power or torque is transmitted from one rotatable shaft to another, the shafts should ideally be in precise alignment. Proper alignment of the shafts reduces the performance requirements imposed on the coupling mechanism and optimizes the operation of the shafts and the machinery to which each shaft is connected. Although the two shafts may be precisely aligned when first installed, it is practically impossible to maintain such an alignment. This is particularly true when the shafts are incorporated in a moving structure such as a motor vehicle or an aircraft. Flexing of the body of the motor vehicle or aircraft may cause the mounting points or journals for the shafts to move relative to one another. Vibrations due to mass imbalances may cause the shafts to move out of alignment, just as may wearing of the journal bearings and wearing of the shafts themselves. In many installations, therefore, the drive coupling that connects the two shafts must be constructed to accommodate significant angular misalignments of the shafts.
A Hooke's or cardan type joint or coupling will permit the transmission of torque or power from one rotatable shaft to another despite angular misalignment of the shafts. Nonetheless, as larger and more powerful engines have been developed to turn shafts at higher rotational speeds, the loads to which shafts may be subjected at start-up of an engine or during operation, due to variations in rotational speed, have substantially increased. To protect drive shafts and the connections between shafts against such shock and vibration loads, many universal couplings of the Hooke's type incorporate resilient bushings. Thus, as is shown in Moulton et al. U.S. Pat. No. 2,975,621, a Hooke's type coupling may include a four-armed spider with an annular bushing of elastomeric material encircling each arm of the spider. Each pair of opposed arms of the spider is connected to a different shaft.
In a coupling such as shown in the Moulton et al. patent, the torque exerted by the driving shaft on the driven shaft produces a combination of compression and tension loads on each elastomeric bushing. Relative axial movement between the shafts (i.e., relative movement between the shafts along a common axis) also loads the bushings in a combination of compression and tension, while angular misalignment of the shafts loads the bushings primarily in torsional shear. Although the elastomeric bushings provide protection against shock and vibration loads, high torque-induced compression loads on the bushings produce large strains in the elastomer. These large strains, which appear as bulging of the elastomer, tend to attenuate the service life of the coupling through elastomer fatigue. The bulging can be reduced by introducing annular metal shims into the bushings, but such shims are expensive to manufacture and add significantly to the cost of the coupling. The coupling is also able to accommodate only limited axial movement between shafts that are coupled together. The addition of metal shims to the bushings of the coupling to improve the fatigue life of the bushings further limits the relative axial movement that can be accommodated.
Another construction for a resilient universal-type coupling is shown, for example, in Brownstein U.S. Pat. No. 2,157,996. In the Brownstein coupling, power is transmitted from the driving shaft to the driven shaft through pairs of disc-like members which have lugs that project from each member toward the other. The lugs on each disc-like member are circumferentially spaced apart so that the lugs of adjacent members mesh together. Bodies of elastomer are interposed between adjacent lugs to cushion shocks and vibrations and prevent their transmission from one shaft to the other. In such a coupling, the driving torque exerts compression loads on the bodies of elastomer. Relative axial movement between the shafts tends to cause shear loads, if any loads, on the bodies of elastomer. ("Anti-friction" strips interposed between each body of elastomer and adjacent metal parts may eliminate all loads on the bodies of elastomer.) Angular misalignment of the shafts exerts a variable combination of shear and "cocking" loads on the bodies of elastomer. The "cocking" loads represent a particular combination of shear and compression loads that results from tilting or cocking a flat surface relative to an elastomeric body bonded to the surface.
As compared to the coupling of the Moulton et al. patent, the Brownstein coupling offers the capability of accepting larger torque loads. The Brownstein coupling can also accommodate substantially greater relative axial movement between shafts, depending upon the clearances provided between the opposed disc-like members. On the other hand, the loads that result from angular misalignment of the coupled shafts will adversely affect the service life of the Brownstein coupling. In particular, the loads on each body of elastomer will vary cyclically in magnitude, direction and type (e.g., shear and cocking) during each revolution of the shafts. Such cyclical loads, particularly the cyclical compression loads that are a component of cocking loads, adversely affect the fatigue life of the elastomeric bodies, as compared to steady loads of similar magnitude.
The detrimental cyclical cocking loads found in the Brownstein coupling are common to couplings of similar construction, such as the coupling of Dossier U.S. Pat. No. 3,902,333. Nonetheless, Peterson U.S. Pat. No. 3,257,826 describes and illustrates a coupling which resembles the Brownstein coupling, but which avoids cyclical cocking loads on the elastomer in the coupling. In the Peterson coupling, each elastomeric body is a complex laminated structure of elastomer and metal. One portion of each laminated elastomeric body is, in effect, a ball-and-socket type joint. Angular misalignment of two shafts connected by such a coupling thus produces only torsional shear loads on the elastomeric bodies. Although the shear loads are cyclical, they are less detrimental to the fatigue life of the elastomeric bodies than cyclical cocking or compression loads. The complexity of the laminated structure in the Peterson coupling adds significantly to the weight and expense of the coupling. Thus, despite the improved fatigue life of the coupling, it is necessarily limited to installations in which the weight can be accommodated and the cost can be justified.